Volume of Gas Hydrate

Keeping in mind the qualification at the end of the previous section, we will consider a map of the distribution of the relative amount of gas hydrate within the sediments of the U.S. Atlantic continental slope and rise south of Hudson Canyon (Fig. 3). The map indicates the volume of hydrate by isopach contours that show the thickness of hydrate that would appear if all hydrate were extracted from the sediment pores and piled on the sea floor.

South Atlantic Margin Profile

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Figure 3. Contours of the volume of gas hydrate existing within the sediments of the U.S. Atlantic continental margin. Volume is indicated as a thickness in meters of the total amount of pure hydrate existing in the pore space. Volumes are estimated by mapping the extent of blanking in seismic profiles in the depth region where hydrate is stable, and using a modeling approach to relate the blanking to proportion of gas hydrate in the sediment.

The map shows that the greatest concentrations of gas hydrate are present south of 34°N, where we identify three major concentrations; a fourth concentration occurs at the northern end of our survey area. One occurs in very deep water, greater than 5000 m, between 71.5° and 74°W on the Lower Rise Hills, which are sediment waves built on the Hatteras Outer Ridge (Mountain and Tucholke, 1985; EEZ Scan 87, 1991). A second concentration occurs on the Blake Ridge, mainly from 75° to 76°W and 31° to 32.5°N (Kvenvolden and Barnard, 1983; Dillon and Popenoe, 1988, Paull et al., 1996). The third southern concentration is contiguous with the Blake Ridge hydrates and extends north-

northeastward from 31.5°N, 77°W to 34° N, 75.5°W; it is labeled 'Carolina Trough Diapirs'(Dillon et al. 1983).

North of 34°N, a series of relatively small, weak concentrations extend along the upper rise to 38°N. At the northeast corner of the map, an extensive, but relatively weak concentration of hydrate, labeled Hudson-Wilmington drape area, covers the region from approximately 37° to 39°N and 71° to 72°W.

The modeling requires considerable development, but in order to provide an example of the possible amount of hydrate gas implied by Figure 3, we calculated the amount of gas hydrate in the Blake Ridge concentration within the small area inside the main 30 m contour on the Blake Ridge (~31.5°-32°N, 75°-76°W) (Dillon et al. 1993). The area within this 30 m contour is approximately 3000 km2. The calculated volume of hydrate within this area, 1.13x10" m3, would contain a volume of methane at STP approximately equal to 1.8xl013 m3, or 645 TCF (trillion cubic feet) of methane. For comparison, natural gas consumption of the United States is recently (1997-1999) at a level of about 21-22 TCF/year.

Other estimates of gas volume in the Blake ridge area have been made as indicated in Table 1 (discussed in Collett and Ladd 2000). The methods have varied widely from seismics to sampling to well logging approaches. The assumptions and areas considered vary; Dillon, for example, took a small area where he considered the concentration to be highest. The result is a set of numbers that vary a great deal, but consider the column that indicates volume of gas hydrate per unit area. With our present state of knowledge, the fact that these numbers are all within a factor of 10 is actually rather encouraging.

Total est. gas Area, Vol./unit area Reference

18 3,000 6 Dillon etal. 1993

80* 100,000 0.8 Holbrook etal. 1996

70* 26,000 2.7 Dickens et al. 1997

37.7 26,000 1.5 Collett and Ladd 2000

57* 26,000 2.2 Collett and Ladd 2000

* includes gas beneath gas hydrate seal

Table 1. Estimates of gas volume that have been made in the Blake Ridge area.

3.2. Thickness of the Zone of Hydrate Stability A map of the depth of the BSR below the sea floor is shown in Figure 4; this interval - from sea floor to BSR - represents the zone in which gas hydrate is stable. Because the thermal gradient is fairly constant over the continental rise region, and because gas hydrate becomes stable to higher temperatures as pressure increases, one would expect a simple pattern in which the BSR would become uniformly deeper as the water becomes deeper (see Chapter 6, Fig. 3). Clearly the pattern is far more complicated. A general trend of greater BSR depth with deeper water is shown in a plot of data (Fig. 5) from several profiles in the Blake Ridge area (Dillon and Paull, 1983), but the scatter is great. In some places, like the Hudson-Wilmington drape area (Fig. 4), the tendency is reasonably pronounced to proceed from shallow sub-bottom BSR to deep as water depth increases. In mass movement areas, such as the Cape Lookout and Cape Fear slides, a distinct thinning of the hydrate zone is associated with the slide scars. Extreme complexity of the thickness of the hydrate-stable zone occurs in the Carolina Trough diapir area.

Figure 4. Contours in meters of the thickness of the layer of hydrate stability within the sediments of the U.S. Atlantic continental margin. This is the distance from the sea floor to the BSR. The BSR represents the bottom of the zone of phase stability for gas hydrates.

Figure 4. Contours in meters of the thickness of the layer of hydrate stability within the sediments of the U.S. Atlantic continental margin. This is the distance from the sea floor to the BSR. The BSR represents the bottom of the zone of phase stability for gas hydrates.

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Figure 5. Plot of the depth of the BSR below the sea floor versus water depth for several profiles in the Blake Ridge area (from Dillon and Paull, 1983).

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Figure 5. Plot of the depth of the BSR below the sea floor versus water depth for several profiles in the Blake Ridge area (from Dillon and Paull, 1983).

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